There’s been a lot of debate about how effective the Bush-era border barrier has been at keeping out illegal crossers and drug smugglers. Some data indicates the barriers have encouraged people to cross in places where there isn’t one, and handprints on the existing border wall show that a determined person can still easily scale it. What the border fence has kept out instead, according to environmentalists, scientists, and local officials, is wildlife. Experts say that if President Donald Trump makes good on his promise to turn the border fence into a continuous, 40-foot concrete wall, the situation for wildlife along the border — one of the most biodiverse areas in North America — will only get worse. A mix of vehicle barriers and pedestrian fencing currently covers only about one-third of the nearly 2,000-mile U.S.-Mexico border – but even with all those gaps, the barriers have made it harder for animals to find food, water, and mates. Many of them are already endangered.

Muddy handprints cover the rusty, iron posts on this section of border fence in far South Texas. The 18-foot-tall barrier, which runs between a national wildlife refuge and a local nature center, ends abruptly less than a mile down the road. Still, somebody clearly thought it was best to cross here.

“This is probably one of the most visible places they could have climbed,” Scott Nicol, co-chair of the Sierra Club’s Borderlands Campaign, said before snapping photos of the handprints. “I don’t know if they got caught or not, but they made it up and over for sure.”

There’s been a lot of debate about how effective the Bush-era barrier has been at keeping out illegal crossers and drug smugglers. Some data indicates the barriers have encouraged people to cross in places where there isn’t one. But the handprints show that a determined person can still easily scale it.

What the border fence has kept out instead, according to environmentalists, scientists and local officials, is wildlife. And the people who have spent decades acquiring and restoring border habitat say that if President Donald Trump makes good on his promise to turn the border fence into a continuous, 40-foot concrete wall, the situation for wildlife along the border — one of the most biodiverse areas in North America — will only get worse.

Right now, a mix of vehicle barriers and pedestrian fencing covers only about one-third of the nearly 2,000-mile U.S.-Mexico border. Even with all those gaps, experts say the barriers have made it harder for animals to find food, water and mates. Many of them, like jaguars, gray wolves and ocelots, are already endangered.

Aaron Flesch, a biologist at the University of Arizona, said most border animals are already squeezed into small, fragmented patches of habitat.

“If you just go and you cut movements off,” he said, “you can potentially destabilize these entire networks of population.”

Still, the impacts of the border fence on wildlife aren’t totally understood. That’s in large part because Congress let the U.S. Department of Homeland Security ignore all the environmental laws that would’ve required the agency to fully study how the barrier would affect wildlife.

Flesch and other scientists say the federal government also has made almost no research money available to support independent studies. Most of the studies that have been done are limited in scope, but their findings are pretty clear: Impeding animal movements puts them on a faster path to extinction.